Exhaust gas recirculation (EGR) is a technique used in diesel engines to achieve low emission of nitrogen oxides (NOx). An EGR technique introduces a portion of engine exhaust gas back to the engine cylinders. The carbon dioxide (CO2) introduced through EGR works as a dilutant and a heat absorbing component in the combustion gas that reduces combustion temperature and consequently lowers NOx emission. Higher CO2 concentration in the combustion gas leads to lower NOx emission and higher hydrocarbon (HC) emission. CO2 concentration in the combustion gas is therefore a factor affecting diesel combustion emissions.
Fuel injection timing is another factor that affects the emission levels of a diesel combustion engine. Advanced injection timing normally results in higher cylinder pressure and temperature and therefore leads to lower HC emissions and higher NOx emissions. Retarded injection timing may result in incomplete combustion of fuel and therefore higher HC emissions and lower NOx emissions. Optimal fuel injection timing is determined based on the condition of the combustion gas (e.g. CO2 concentration). A mismatch between injection timing and combustion gas condition generally leads to poor emissions.
Diesel combustion control calibration takes into account both CO2 concentration and injection timing. For each engine operating point (engine speed vs. load), a target combustion gas condition (i.e. CO2 concentration) is determined by specifying a target mass air flow (MAF). The fuel injection timing is calibrated under the target combustion gas condition for optimized emissions. In steady state operations, reaching the target MAF provides for the target combustion gas condition. During transient operations of the engine, the actual combustion gas condition often deviates from the target condition even if the engine is following the target MAF. Since engine control systems use the fuel injection timing calibrated for the steady state condition, a mismatch between fuel injection timing and combustion gas condition occurs during transient diesel engine operation. The mismatch leads to high emission levels of NOx and HC.